Hydrophone with automatic inhibition in case an adjustable immersion threshold is exceeded

ABSTRACT

A hydrophone which is automatically inhibited when the immersion depth exceeds a predetermined threshold by action, on the push element ( 7 ) of a switch ( 6 ), of a flexible wall that bends under the action of the outside pressure. The hydrophone comprises a tubular body ( 1 ) with two chambers ( 1   a   , 1   b ) separated by a partition ( 2 ). One of the chambers is closed by a flexible diaphragm ( 4 ). Switch ( 6 ) is fixed at the center of one of the chambers. Diaphragm ( 4 ) can push down on and actuate push element ( 7 ) by bending. The body can advantageously consist of two parts that can be screwed onto one another, which allows easier adjustment of the inhibition pressure. A pressure-sensitive reception unit (S) is arranged in the other chamber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hydrophone with automatic inhibitionwhen the immersion depth exceeds a certain adjustable threshold.

2. Description of the Prior Art

It is well-known to form hydrophones by assembling elements sensitive topressure variations, such as discs made of a piezoelectric material,associated each with a pair of electrodes arranged on either side. Eachdisc is stuck onto a flexible support such as a diaphragm, one of thefaces thereof being exposed to the pressure variations to be measured.

The flexible support has for example the shape of a cup which can besupported by a rigid housing or which can rest against an identical cup,itself carrying one or more sensitive elements, the two cups being incontact with one another at one edge and delimiting a housing. Theelectrodes of the two sensitive elements are preferably electricallyinterconnected so as to provide compensation for the parasitic effectsdue to accelerations.

When the outside static pressure increases, the two plates bend untilthey rest against each other. The space between them is so selected thattheir maximum deformation, when they are pressed against each other,remains within elastic deformation limits. The detector is thusprotected against accidental overpressures. The housing thus formed canbe coated with a layer made of a material transparent to acoustic waves.

The sensitive elements can be externally fastened to the housing so thattheir sensitivity does not vary much with the hydrostatic pressurevariations (less than 10% for a 10 Map static pressure for example). Thesensitive elements are generally covered with a protecting coating (suchas a varnished araldite layer) so as to maintain a sufficient electricinsulation in relation to the outside environment. This layout allowsobtaining of very sensitive detectors at a relatively low cost.

According to another well-known layout, the sensitive elements arefastened to the inner faces of the cups and therefore inside thehousing, which provides good protection against the outside environment.A stop can be arranged between the two cups so as to limit deformationof the diaphragms towards the inside and to short-circuit the electrodesof the two sensitive elements facing each other.

It is also well known to position the housings containing one or twosensitive elements inside a rigid tube by means of a hollowed flatcentering element made of a deformable material, and to run anacoustically transparent sealed sheath therein. In subsea seismiclistening applications or for seismic prospecting, these tubes aredistributed in large numbers inside a supple sheath of often very greatlength, or streamer, filled with kerosine or mineral oil, which is towedby a boat

Various piezoelectric detectors are described for example in Frenchpatents 1,556,971, 2,122,675, 2,748,183 or 2,792,802 or U.S. Pat. No.5,889,731, all filed in the name of one of the applicants or of both, orin U.S. Pat. Nos. 3,970,878, 4,336,639, 4,926,397 or 5,136,549.

Piezoelectric hydrophones, such as those mentioned above, are designedto work within a certain depth range. The sensitivity of thepiezoelectric elements decreases with the flexion of the diaphragms.Beyond a certain flexion, their response to the pressure variations tobe measured stops being reliable, and it can even become zero in case ofa short-circuit of the electrodes facing each other (detectors withsensitive elements against the inner faces of the cups).

SUMMARY OF THE INVENTION

The automatic-inhibition hydrophone according to the invention is suitedfor use in a well-defined and adjustable depth range and isautomatically inhibited when a predetermined depth threshold isexceeded. The hydrophone of the invention comprises at least onedetection unit producing an electric signal on electric wires inresponse to the pressure variations applied.

The hydrophone of the invention comprises at least one flexiblediaphragm that bends under the action of the pressure exerted externallyon the hydrophone and a switch arranged opposite so as to be actuated bythe diaphragm for a predetermined outside pressure, at least one of theelectrical wires of the detection unit being connected to the switch sothat the release of the switch inhibits the response of the sensitiveelement.

According to a first embodiment, the hydrophone comprises a tubular bodycomprising a fist chamber closed by a flexible diaphragm which bendsunder the action of the pressure exerted externally on the hydrophone,and a second chamber open onto the outside environment and communicatingwith the first chamber by a channel. The switch has a push element whichis arranged opposite the flexible diaphragm closing the first chamber soas to be actuated thereby for a predetermined outside pressure, thedetection unit is arranged in the second chamber, so that at least oneof the electrical wires is connected to the switch in the first chamber,so that release of the switch by action of the flexible diaphragminhibits the response of the sensitive element.

The hydrophone comprises for example a block made of an acousticallytransparent material in which the detection clement of the secondchamber is embedded.

According to a first variant, the second chamber is separated from thefirst chamber by an inner partition including at least one channelallowing passage of at least one electrical wire connecting the switchto the detection unit, and the inner wall of the body on the firstchamber side is, for example, designed to limit deformation of thediaphragm towards the inside of the first chamber.

According to another variant, the second chamber is separated from thefirst chamber by an inserted rigid plate made of a dielectric materialwhich is fastened to the inside of the body, this plate carrying theswitch and being suited to bring it into electrical contact with atleast one electrical wire connecting the switch to the detection unit.

This rigid plate is, for example, an insulating plate provided withconducting tracks allowing connection of the terminals of the switch tothe electric wires of the detection unit.

The hydrophone comprises, for example, a ring made of a plastic ormetallic material, arranged in the first chamber between the diaphragmand the rigid plate, which is suited to limit deformation of thediaphragm towards the inside of the first chamber.

According to a first embodiment example, the detection element in thesecond chamber comprises a housing of two cups resting against eachother, at least one of the cups being provided with a flexible centralpart or diaphragm, and at least one sensitive element associated withelectrodes, which is fastened to the central part of at least one of thecups, electric conductors connected to the electrodes of each sensitiveelement, and a protective sheath for protection of the detection unit,the switch being connected to at least one electrical conductor.

According to a second embodiment example, the detection element in thesecond chamber comprises a housing of two cups resting against eachother, at least one of the cups being provided with a flexible centralpart or diaphragm, and at least one sensitive element associated withelectrodes, which is fastened to the central part of at least one of thecups, electrical conductors connected to the electrodes of eachsensitive element, and a protective sheath for protection of thedetection unit, the switch connected to at least one electric conductorbeing arranged inside the housing so as to be released by at least onediaphragm of the housing bending under the action of the hydrostaticpressure.

According to another example, the detection element comprises anintermediate support arranged between the two cups, the switch beingfastened to this intermediate support with a push element thereof facingthe central part of one of the cups, and a sealed insulating bushing forpassage, towards the outside of the housing, of an electric conductorconnected to the switch.

According to another embodiment example, the switch is of the typeprovided with flexible blades suited to remain in electric contact withone another as long as the flexion of at least one of the diaphragmsremains below a fixed threshold.

According to another embodiment, the tubular body of the hydrop honecomprises a fixed part containing the detection unit and the switch, anda moving part closed at a first end by the diaphragm, tightly screwedonto the fixed part, so that the effective space remaining between thediaphragm and the push element allows the switch to be always releasedat a well-defined pressure.

The hydrophone is therefore preferably mounted in such a way that themoving part of the body is first fitted onto the fixed part by screwing,a predetermined nominal pressure applied to the diaphragm causingbending thereof towards the inside of the body, then fitting of the twoparts onto each other by continued screwing until the switch is actuatedby the bent diaphragm.

As the case may be, the switch is normally open or normally closed.

The hydrophone, as defined above, is light and compact. Its outersurface is practically free of rough patches likely to disturb fluidflows around the outer surface. The diaphragm provides perfect sealingand insulates the switch electrically. Furthermore, the embodiment andthe mounting mode wherein the body is made of two parts that fit intoone another allows obtaining a good reproducibility of the hydrophonetriggering conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the hydrophone according to theinvention will be clear from reading the description hereafter of nonlimitative examples, with reference to the accompanying drawingswherein:

FIG. 1 diagrammatically shows a first embodiment of the hydrophone withits switch allowing controlled inhibition when a fixed depth value isexceeded,

FIG. 2 diagrammatically shows an embodiment of the detection element ofthe hydrophone,

FIG. 3 diagrammatically shows a protective sheath for the detectionelement,

FIG. 4 diagrammatically shows the detection element of FIG. 3 in itstubular housing,

FIG. 5 diagrammatically shows a second embodiment of the detectionelement with an internal switch,

FIG. 6 diagrammatically shows a third embodiment of the detectionelement with an internal switch provided with flexible blades,

FIGS. 7 and 8 diagrammatically show two embodiment variants of thehydrophone of FIG. 1 comprising inserted elements inside the body of thehydrophone and two different switches,

FIG. 9 diagrammatically shows an improved embodiment allowing obtainingof precise adjustment of the hydrophone triggering threshold, and

FIG. 10 diagrammatically shows a hydrophone mounting tool allowingobtaining of fine adjustment of the hydrophone triggering threshold.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The hydrophone comprises (FIGS. 1 and 4) a tubular body 1 open at bothends. A partition 2 divides the inside of body 1 into two unequalchambers 1 a and 1 b which communicate with one another through arelatively narrow channel 3. The end of body 1, on the small chamber 1 aside, is closed by a diaphragm 4 made for example from a Cu/Be alloy.The terminal part of body 1 is bevelled towards the inside. The smallbowl 5 defined by this bevel allows diaphragm 4 to bend towards theinside of body 1. A switch 6 provided with a push element 7, on whichdiaphragm 4 can push and actuate by bending, is fastened (by adhesionfor example) to the center of small chamber 1 a. Switch 6 can benormally open or normally closed, as the case may be.

A pressure-sensitive reception unit S is arranged in the larger chamber1 b. The unit S is embedded in a block made of an acousticallytransparent plastic material 8 which insulates it from the outsideenvironment. The signal produced by reception unit S in response to thepressure variations of the outside environment is available onelectrical conductors 9 a and 9 b. Switch 6 is interposed on at leastone of the conductors so that its release inhibits the signal ofreception unit S. If switch 6 is normally closed, for example (FIG. 1),it is interposed on one of the wires 9 a so that its release opens thecircuit.

The flexibility of diaphragm 4 and the position of push element 7 are soselected that switch 6 is actuated and the hydrophone is cut off for apredetermined hydrostatic pressure, 3 MPa for example. It has beenobserved that the hydrostatic triggering pressure remains particularlystable and practically insensitive to aging. Reception unit S can be ofany type, with for example having a sensitive element of a tube or ahollow sphere made of a piezoelectric ceramic associated withelectrodes. The pressure variations are transmitted thereto creatingmechanical stresses that the sensitive element translates into anelectric signal.

In the embodiment described hereafter by way of example, the flexion ofthe diaphragms is used to generate stresses in one or more piezoelectricceramic discs.

Each reception unit S comprises (FIG. 2) a housing 10 having twoidentical cups 10 a and 10 b arranged symmetrically, resting againsteach other on the periphery thereof, provided for example with a restingedge or rim 11. The cups can be machined or drawn. A sensitive element12, such as a piezoelectric ceramic disc for example associated with twoelectrodes 12 a and 12 b, adheres to the face of the flexible centralpart of each cup 10 a and 10 b. Electrode 12 b of each sensitive elementis for example a conducting film interposed between sensitive disc 5 andthe resting face of cup 10 a and 10 b, or possibly this face itself, ifelectrode 12 b is electrically conducting. Preferably, according to aconventional connection mode, electrodes 12 a and 12 b of the twosensitive elements 12 are respectively interconnected. When housing 10is electrically conducting (as it is the case in the embodimentdescribed), it is this housing which provides interconnection ofelectrodes 12 b in contact with cups 10 a and 10 b. The oppositeelectrodes 12 a are interconnected by connection of associatedconductors 13 a A conducting wire 13 b is electrically connected tohousing 10. The voltage generated by the sensitive elements in responseto the pressures applied outside the housing is available between wires13 a and 13 b. Housing 10 is inserted in protective sheath 14 made of aninsulating plastic material.

Reception unit 1 in its sheath 14 is in a median plane of the largerchamber 1 b (FIG. 4) and embedded in a block of controlled thickness.

According to the embodiment of FIG. 5, the hydrophone comprises areception unit 1 as shown in FIG. 2, directly associated with a switch15 arranged inside housing 2. The switch is fastened to an intermediatesupport 17 arranged between cups 10 a and 10 b. The switch is normallyclosed and its terminals are interposed on electric conductor 13 bconnected to the housing. This conductor 13 b runs through the wall ofhousing 10 by means of an insulating sealed bushing 18 (glass bead forexample). The voltage V delivered by the hydrophone is available betweeninterconnected conducting wires 13 a and common wire 13 b. Again, inthis case, the height of cups 10 a and 10 b and their flexibility areselected according to the dimensions of switch 15 so that push element16 is actuated for a well-defined boundary hydrostatic pressure, and thehydrophone is disconnected.

According to the embodiment of FIG. 6, switch 6 is of the type providedwith flexible blades with a first blade 19 electrically connected to oneof the cups 10 a and a second blade 20 electrically insulated fromhousing 10 by a glass bead 16. Blades 19 and 20 are designed to remainin contact with one another as long as the flexion of at least one ofthe diaphragms under the action of the outside pressure remains withinfixed limits. The absence of contact brings, for example, one of theterminals of the hydrophone into the air.

In the embodiment of the hydrophone shown in FIGS. 7 and 8, the elementscorresponding to those of FIGS. 1 and 4 have the same reference numbers.The hydrophone also comprises a tubular body 1 with one end closed by adiaphragm 4 made of for example a Cu/Be alloy. A ring 21 made of aplastic material such as nylon and bevelled towards the inside ispressed against diaphragm 4. The small bowl 5 defined by this bevelallows flexion of diaphragm 4 towards the inside of body 1. Thepartition, which separates chambers 1 a and 1 b, is an inserted rigidplate 22 made of a non-conducting material. Switch 6 is provided with apush element 7 fastened to one of the faces, on the small chamber 1 aside. By means of conducting tracks running through plate 22, theterminals of switch 6 are electrically connected to electricalconductors 9 a and 9 b associated with transducer S.

Mounting of the hydrophone is thus simplified. Ring 21 is first fittedinto body 1 at the open end thereof until it is in contact with thediaphragm and it is fastened to the inner wall of the body, by adhesionfor example. Rigid plate 22 is then engaged until the push element is incontact with the diaphragm and the push element is similarly fastened byadhesion so as to tightly separate the two chambers. Transducer S, whichis electrically connected to at least one of the conducting wiresconnected through rigid plate 22 to the terminals of switch 6, is thenplaced in the body and is insulated from the outside environment byfilling large cavity 1 b with an acoustically transparent material 8.

Similarly, switch 6 can be normally open or normally closed.

According to the embodiment of FIG. 9, which is a variant of theprevious embodiment of FIG. 7, body 23 of the hydrophone comprises apart 23 a containing reception unit S and associated elements 6, 7 and22, and a complementary part 23 b forming a cover which can be assembledby screwing onto part 23 a. The inner wall of part 23 a comprises ashoulder 24 against which rigid plate 22 rests and an external thread25. Cover 23 b also comprises an inner shoulder on which the bevelledring forming a stop 21 rests and an external thread suited to theexternal thread 25 of part 23 a of the body. Diaphragm 4 is welded(laser welding for example) onto the terminal face of cover 23 b.Orifices 26 are provided, preferably through the wall of the body,allowing discharge of the air that may have been trapped during mountingof the hydrophone.

During the mounting operation, rigid plate 22 associated with receptionunit S is fitted into part 23 a of the body until it rests againstshoulder 24 and it is held in position by drawing. Cover 23 b is thenscrewed onto part 23 a so as to insulate chamber 1 a from the outsideenvironment. Driving in of the cover by screwing is so adjusted that thehydrophone is inhibited for a predetermined pressure.

A tubular part 27 (FIG. 10), whose inner section is suited to the outersection of cover 23 a and internally provided with a seal 28, istherefore used. A channel allowing communication with a sourcedelivering a fluid under an adjustable pressure, adjusted to the desiredhydrophone inhibition pressure, is connected to part 27. Tubular part 27allows cover 23 b of the body to be tightly covered and thepredetermined pressure is applied, which causes the diaphragm to bendtowards the inside of the housing. By means of a tool, cover 23 b isthen screwed onto part 23 a of the body until the bent diaphragm ispressed against push element 7 and releases switch 6.

This adjustment of each hydrophone is particularly simple, fast andconvenient because it is suited to the mechanical elements used whichcooperate to produce the release. It accounts for the effectiveflexibility of diaphragm 4, of the effective dimensions of switch 6, ofthe effective travel of push element 7, considering the manufacturingtolerances of these elements, and obtains release at a constant nominalpressure.

For applications to marine seismic prospecting, hydrophones such asthose described above can for example be distributed at a distance fromone another along a streamer suited to be towed immersed behind atowboat.

What is claimed is:
 1. A hydrophone with automatic inhibition of sensing of pressure variations when a fixed outside depth threshold is exceeded, comprising: at least one detection unit producing an electrical signal on electrical wires in response to applied pressure variations the at least one detection unit being enclosed in a body provided with at least one flexible wall flexing under an action of pressure exerted externally on the hydrophone, a switch which is actuated by the at least one flexible wall acting on the switch when a fixed predetermined outside pressure is reached and at least one of the electrical wires of the at least one detection unit are connected to the switch and wherein activation of the switch inhibits an output of the electrical signal from the at least one detection unit.
 2. A hydrophone in accordance with claim 1, comprising: a tubular body comprising a first chamber closed by a flexible diaphragm flexing under action of pressure exerted externally on the hydrophone, the flexible diaphragm forming the flexible wall, and a second chamber open to an outside environment and insulated from the first chamber, the switch being provided with a push element in the first chamber opposite the flexible diaphragm so as to be actuated by the flexible diaphragm for a predetermined outside pressure, the at least one detection unit being in a second chamber, at least one electrical wire being connected to the switch in first chamber so that release of the switch through flexing of the flexible wall inhibits response of the at least one detection unit.
 3. A hydrophone in accordance with claim 2, comprising: a block made of an acoustically transparent material in which detection unit of second chamber is embedded.
 4. A hydrophone in accordance with claim 2, wherein: the second chamber is separated from the first chamber by an inner wall including at least one channel allowing passage of at least one electrical wire connecting the switch to the at least one detection unit.
 5. A hydrophone in accordance with claim 3, wherein: the second chamber is separated from the first chamber by an inner partition including at least one channel allowing passage of at least one electrical wire connecting the switch to the at least one detection unit.
 6. A hydrophone in accordance with claim 4, wherein: the inner wall of the body on a side of the first chamber limits deformation of the diaphragm towards an inside of first chamber.
 7. A hydrophone in accordance with claim 5, wherein: the inner wall of the body on a side of the first chamber limits deformation of the diaphragm towards an inside of first chamber.
 8. A hydrophone in accordance with claim 2, wherein: the second chamber is separated from the first chamber by a dielectric rigid plate which is fastened to a inside of the body, the plate carrying the switch and bringing the switch into electrical contact with at least one electrical wire connecting the switch to the at least one detection unit.
 9. A hydrophone in accordance with claim 3, wherein: the second chamber is separated from the first chamber by a dielectric rigid plate which is fastened to a inside of the body, the plate carrying the switch and bringing the switch into electrical contact with at least one electrical wire connecting the switch to the at least one detection unit.
 10. A hydrophone in accordance with claim 8, wherein: the plate is an insulating plate provided with conducting tracks which electrically connect terminals of the switch to electrical wires of the at least one detection unit.
 11. A hydrophone in accordance with claim 9, wherein: the plate is an insulating plate provided with conducting tracks which electrically connect terminals of the switch to electrical wires of the at least one detection unit.
 12. A hydrophone in accordance with claim 8, comprising: a ring, made of one of a metal or a plastic material, disposed in first chamber between the at least one flexible wall and the rigid plate which limits deformation of the at least one flexible wall towards an inside of first chamber.
 13. A hydrophone in accordance with claim 9, comprising: a ring, made of one of a metal or a plastic material, disposed in first chamber between the at least one flexible wall and rigid plate which limits deformation of the at least one flexible wall towards an inside of first chamber.
 14. A hydrophone in accordance with claim 10, comprising: a ring, made of one of a metal or a plastic material, disposed in first chamber between the at least one flexible wall and rigid plate which limits deformation of the at least one flexible wall towards an inside of first chamber.
 15. A hydrophone in accordance with claim 11, comprising: a ring, made of one of a metal or a plastic material, disposed in first chamber between the at least one flexible wall and rigid plate which limits deformation of the at least one flexible wall towards an inside of first chamber.
 16. A hydrophone in accordance with claim 2, wherein the at least one detection unit in second chamber comprises: a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 17. A hydrophone in accordance with claim 3, wherein the at least one detection unit in second chamber comprises: a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 18. A hydrophone in accordance with claim 4, wherein the at least one detection unit in second chamber comprises: a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 19. A hydrophone in accordance with claim 5, wherein the at least one detection unit in second chamber comprises: a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 20. A hydrophone in accordance with claim 6, wherein the at least one detection unit in second chamber comprises: a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 21. A hydrophone in accordance with claim 7, wherein the at least one detection unit in second chamber comprises: a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 22. A hydrophone in accordance with claim 8, wherein the at least one detection unit in second chamber comprises: a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 23. A hydrophone in accordance with claim 9, wherein the at least one detection unit in second chamber comprises: a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 24. A hydrophone in accordance with claim 10, wherein the at least one detection unit in second chamber comprises: a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 25. A hydrophone in accordance with claim 11, wherein the at least one detection unit in second chamber comprises: a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 26. A hydrophone in accordance with claim 12, wherein the at least one detection unit in second chamber comprises: a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 27. A hydrophone in accordance with claim 13, wherein the at least one detection unit in second chamber comprises: a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 28. A hydrophone in accordance with claim 14, wherein the at least one detection unit in second chamber comprises: a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 29. A hydrophone in accordance with claim 15, wherein the at least one detection unit in second chamber comprises: a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 30. A hydrophone in accordance with claim 2, comprising: a second chamber; and wherein the at least one detection unit is located in the second chamber and comprises a housing including two cups resting against one another, at least one of the two cups being provided with a flexible central part forming a flexible diaphragm, and at least one sensitive element associated with electrodes, which is fastened to the flexible central part of at least one of cups, electrical conductors connected to electrodes of each sensitive element, and a protective sheath for the at least one detection unit and the switch is connected to the at least one electrical.
 31. A hydrophone in accordance with claim 30, wherein: the switch is fastened to an intermediate support between the two cups of the housing.
 32. A hydrophone in accordance with claim 30, wherein: the switch is fastened to an intermediate support between the two cups of housing, the switch being fastened to the intermediate support with a push element of the switch facing the flexible diaphragm of one of the two cups, and an insulating and sealed bushing allowing passage, out of the housing, of an electrical conductor connected to the switch.
 33. A hydrophone as claimed in claim 30, wherein: the switch has flexible blades with a first blade electrically connected to one of the cups and the blades remaining in contact with one another as long as flexion of at least one of the diaphragms, under the action of the outside pressure, remains below a fixed limit.
 34. A hydrophone in accordance with claim 2, comprising: a tubular body including a fixed part containing the at least one detection unit and the switch, and a moving part closed at a first end by the at least one flexible wall, connected to the fixed part by screwing, so that space remaining between the at least one flexible wall and a push element of the switch allows the switch to be always released at a pressure.
 35. A method of mounting the hydrophone as claimed in claim 34, comprising: fitting the moving part of the body onto fixed part by screwing, applying to the at least one flexible wall a predetermined pressure causing the at least one flexible wall to bend towards an inside of the body, and continuing to screw the two parts onto one another until the switch is released.
 36. A hydrophone in accordance with claim 1, wherein: the switch is normally open.
 37. A hydrophone in accordance with claim 1, wherein: the switch is normally closed. 